With the practical implementation of blue light-emitting diodes (LED) in recent years, white LEDs that utilize blue LEDs are also being actively developed. White LEDs have low power consumption and extended usable life compared to existing white light sources, for which reason efforts continue to progress toward their deployment in liquid crystal panel backlights, indoor and outdoor lighting devices, and the like.
The white LEDs presently being developed are obtained by applying Ce-doped YAG (yttrium-aluminum-garnet) onto the surfaces of blue LEDs. However, the fluorescence peak wavelength of Ce-doped YAG is around 560 nm, and when this fluorescence color and the light of blue LED are mixed to produce white light, the resulting white light is slightly blue-tinted, such that these types of white LEDs have had poor color rendering properties.
Numerous oxynitride phosphors are being studied in order to deal with this issue, and in particular, Eu-activated α-SiAlON phosphors are known to emit (yellow to orange) fluorescence with a peak wavelength of around 580 nm that is longer than the fluorescence peak wavelength of Ce-doped YAG (see PTL 1), and when a white LED is fabricated by using the α-SiAlON phosphor or by combining it with a Ce-doped YAG phosphor, it is possible to produce a white LED that exhibits a bulb color with a lower color temperature than a white LED using only Ce-doped YAG.
However, for a Ca-containing α-SiAlON phosphor activated by Eu, represented by the general formula:CaxEuySi12−(m+n)Al(m+n)OnN16−n no high luminance phosphor has yet been developed that has practical value.
PTL 2 discloses a phosphor exhibiting excellent luminous efficiency and having a fluorescence peak at a wavelength of 595 nm or longer, and a method for producing it, wherein smooth-surface particles having sizes greater than in the prior art are obtained by adding previously synthesized α-SiAlON powder as seed crystals for grain growth to a starting powder, and powder having a specific particle size is obtained from the synthesized powder without conducting pulverizing treatment.
Specifically, there is disclosed an α-SiAlON phosphor which is an α-SiAlON phosphor having the composition (Ca1.67, Eu0.08) (Si, Al)12(O, N)16 (x+y=1.75, O/N=0.03), the peak wavelength of the fluorescence spectrum obtained upon excitation with blue light of 455 nm being in the range of 599 to 601 nm and the luminous efficiency (=external quantum efficiency=absorptivity×internal quantum efficiency) is from 61 to 63%.
However, the PTL 2 does not give a specific example of a phosphor having a fluorescence peak wavelength of smaller than 599 nm or larger than 601 nm and exhibiting practicable luminous efficiency.
PTL 3 discloses a light-emitting device characterized by using a phosphor containing α-SiAlON as a main component, represented by the general formula: (Caα, Euβ) (Si, Al)12(O, N)16 (provided that 1.5<α+β<2.2, 0<β<0.2, O/N≤0.04) and having a specific surface area of 0.1 to 0.35 m2/g, as well as a vehicle lighting device and a headlamp using the same.
PTL 3 discloses examples of an α-SiAlON phosphor, where the peak wavelengths of the fluorescence spectra obtained upon excitation with blue light of 455 nm are 592, 598 and 600 nm, and the luminous efficiencies (=external quantum efficiencies) are 61.0, 62.7, and 63.2%, respectively.
However, PTL 3 does not give a specific example of a phosphor having a fluorescence peak wavelength of smaller than 592 nm or larger than 600 nm and exhibiting practicable luminous efficiency.
PTL 4 discloses a SiAlON phosphor having a unique property of emitting light with high luminance compared to conventional phosphors, and a method for producing it, wherein a metal compound mixture capable of composing a SiAlON phosphor by firing is fired in a gas at a specific pressure in a specific temperature range, pulverized and classified to a specific particle size, and further heat treated.
However, what is specifically disclosed in PTL 4 is only the peak luminous intensity, and since the peak luminous intensity varies depending on the measuring apparatus and measurement conditions, it is not clear whether a practical level of luminous intensity is obtained.
PTL 5 discloses Li-containing α-SiAlON phosphor particles and a method for producing it, the method comprising: mixing a silicon nitride or nitrogen-containing silicon compound powder, an AlN-containing aluminum source, a Li source and a Eu source; firing the mixture at 1500 to 1800° C. in an inert gas atmosphere containing nitrogen at ordinary pressure, or in a reducing gas atmosphere, to obtain a lithium-containing α-SiAlON powder as a starting material; adding and mixing additional lithium source to the powder; and refiring either at a lower temperature than the firing temperature or at 1100° C. or higher and less than 1600° C., in an inert gas atmosphere containing nitrogen or in a reducing gas atmosphere at ordinary pressure.
However, what is specifically disclosed in PTL 5 is Li-containing α-SiAlON phosphor particles with a peak wavelength of 572 to 588 nm, while no specific examples are provided of phosphors with a fluorescence peak wavelength of greater than 588 nm, and only the peak luminous intensity is disclosed without specific quantum efficiencies, such that it is not clear whether the luminous efficiency is of a practical level.
PTL 6 describes an α-SiAlON phosphor represented by the general formula: LixCayEuzSi12−(m+n)Al(m+n)OnN16−n (x being such that 0<x≤0.8, and 0.3≤m<4.5, 0<n<2.25).
However, what is specifically disclosed in PTL 6 is (Ca, Li)-containing α-SiAlON phosphor particles with a peak wavelength of near 560 nm, whereas no phosphor with a fluorescence peak wavelength of greater than 590 nm is mentioned and the specific quantum efficiencies are not provided, such that it is not clear whether the luminous efficiency is of a practical level.